(1) Field of the Invention
The present invention relates to a composite filter for water purification and filtration. In particular, the present invention comprises a ceramic filter coated with a multi-tapered nanocrystalline, sintered, metal oxide catalyst for degrading remaining ozone in the water used to kill microorganisms.
TABLE 1AbbreviationsDIDeionized waterFPForming precursors or forming potentialTHMTrihalomethaneHAAHaloacetic acidTOCTotal organic carbonNOMNatural organic materialNFNanofiltrationMFMicrofiltrationUFUltrafiltrationDBPDisinfection by-productsHSHumic substancesNHSNon-humic substancesDOCDecomposition Organic CompoundsBDOCBiodegradable Decomposition OrganicProductsMWOMolecular Weight CutoffSWTRSurface Water Treatment RuleP/DBPMicrobial Disinfection By-ProductsROReverse OsmosisSEMScanning Electron Microscope
(2) Description of the Related Art
As a result of the Surface Water Treatment Rule (SWTR) promulgated in 1989 and the Long Term 2 ESWTR Agreement in Principle signed by the US EPA and the members of the Microbiol-Disinfection Byproducts Rule (D/DBP) Cluster Federal Advisory Committee Act (FACA) Committee (65 FR 83015), water utilities are required to achieve up to 2.5-log inactivation and/or removal of Cryptosporidium sp. beyond conventional treatment. As a result, at least 119 utilities in the U.S. are using microfiltration (MF) or ultrafiltration (UF) (USEPA. Low-Pressure Membrane Filtration for Pathogen Removal: Application, Implementation and Regulatory Issues. Office of Water. 815-C-01-001. April 2001 (2001a)) for the removal of pathogens and particles from surface water or groundwater under the influence of surface water.
Membrane filtration is considered as one of the best available technologies (BAT) for meeting the Stage 2 D/DBP requirements (Cleveland, C. T., Big Advantages in Membrane Filtration. Jour. AWWA, 91:6:10-10 (1999); Taylor, J. S., et al., Membranes. In Water Quality and Treatment. 5th Ed., American Water Works Association, Ch. 11 (1999); Arora, H., et al., DBP Occurrence Survey. Jour. AWWA., 89:6:60-68 (1997)). During the last decade, better membranes have been developed and the characterization of membrane surfaces and causes of membrane fouling are better (although not completely) understood (AWWA Membrane Technology Research Committee, Committee Report: Membrane Processes. Jour. AWWA., 90:6:91-105 (1998)). However, one of the major problems with membrane processes are the decrease in permeate flux, resulting from membrane fouling (e.g., see Cho, J., et al., Effects of Molecular Weight Cutoff, f/k ration (a Hydrodynamic condition)., and Hydrophobic Interactions on Natural Organic Matter Rejection and Fouling in Membranes. J. Water Supply Res. and Technol.-AQUA, 51:109-123 (2002); Lee, H., et al., Cleaning Strategies for Flux Recovery of an Ultrafiltration Membrane Fouled by Natural Organic Matter. Water Res. 35:3301-3308 (2001); Field, R. W., Mass Transport and the Design of Membrane Systems. Industrial Membrane Separation Technology, Scott, K. and Hughes, R., Eds., Blackie Academic & Professional, Glasgow, UK (1996)).
Although membrane filtration provides a barrier against pathogens and particles, disinfection after filtration is usually necessary to control bacterial regrowth on the filtrate side of the membrane (US EPA. Ibid; April 2001 (2001a)). As a result, the production of DBPs during the disinfection process can remain problematic (Miles, A. M., et al., Comparison of Trihalomethanes in Tap Water and Blood. Environ. Sci. Techno. 39:1692-1698 (2002); Morris, R. D., et al., Chlorination, Chlorination By-products, and Cancer: A Meta-analysis. Am. Jour. Public Health, 82:7:955-963 (1992); Mughal, F. H., Chlorination of Drinking Water and Cancer: A Review. Jour. Envir. Pathol., Toxicol. & Oncol., 11(5,6):287-292 (1992); Kool, H. J., et al., Mutagenic and Carcinogenic Properties of Drinking Water. In Water Chlorination: Chemistry, Environmental Impacts, and Health Effects, Vol. 5, Jolley, R. L., Brungs, W. A., and Cumming, R. B., Eds., Lewis Pub., Chelsea, Mich. (1985)). As the DBP regulations continue to become more stringent (US EPA40 CFR Parts 9, 141, and 142. National Primary Drinking Water Regulations: Stage 2 Disinfection By-products Rule. Deliberative Draft, Oct. 17, 2001 (2001b); Pontius, F. W., Regulations in 2000 and Beyond. Jour. AWWA., 92:3:40-54 (2000); Pontius, F. W., Complying with Future Water Regulations. Jour. AWWA., 91:46-58 (1999); U.S. EPA. Disinfectants and Disinfection By-products, Final Rule. Fed. Reg., 63:241:69394 (Dec. 16, 1998); Arora, H., et al., (Ibid; 1997), novel ways to further remove DBP precursors in order to meet the more stringent regulations concerning DBPs must be developed.
Membrane Filtration
The membrane process is an effective means for removing turbidity, organics, microorganisms, and DBP precursors to comply with the more stringent regulatory controls. Compared to conventional treatment, membrane processes i) provide high quality water, ii) minimize disinfectant demand, iii) are more compact, iv) provide easier operational control and less maintenance, and v) generate less sludge (Nakatsuka, S., et al., Drinking Water Treatment by Using Ultrafiltration Hollow Fiber Membranes. Desalination, 106:55-61 (1996).
The efficacy of membrane filtration varies widely with membrane type and source water. For example, Lâiné, J. M., et al., Influence of Bromide on Low-Pressure Membrane Filtration for Controlling DBPs in Surface Waters. Jour. AWWA., 85:6:87-99 (1993), found that UF removed less than 15% of the TOC from three surface water samples taken in California and Ontario. Jacangelo, J. G., et al., UF with Pretreatment for Removing DBP Precursors. Jour. AWWA., 87:3:100-112 (1995), reported very low removals of trihalomethane (THM) and haloacetic acid (HAA) precursors and bromide by UF. On the contrary, Taylor, J., et al., Applying Membrane Processes to Groundwater Sources for Trihalomethane Precursor Control. Jour. AWWA., 79:8:72-82 (1987), using two Floridian groundwaters that contained excessive THM precursors, and Fu, P., et al., Selecting Membranes for Removing NOM and DBP Precursors. Jour. AWWA., 86:12:55-72 (1994), using a ground water from southern California that contained high color and TOC, both reported that UF effectively rejected NOM during treatment.
Reverse osmosis (RO) has advantages compared to conventional drinking water treatment, but the costs are higher (Ericsson, B. and Hallmans, B., Membrane Applications in Raw Water Treatment with and without Reverse-Osmosis Desalination. Desalination, 98(1-3): 3-16 (1994). Karakulski, K., et al., Pilot Plant Studies on the Removal of Trihalomethanes by Composite Reverse Osmosis Membranes. Desalination, 140:3:227-234 (2001) reported that over 80% of THMs were retained by RO membranes.
Several studies have demonstrated large removals of DBP precursors using nanofiltration (NF). Lâiné, J. M., et al., (Ibid; 1993) reported that NF membranes were effective at removing DBP precursors, but ineffective at removing bromide. Siddiqui, M., et al., Membranes for the Control of Natural Organic Matter from Surface Waters. Wat. Res., 34:13:3355-3370 (2000) found that NF was much more effective at rejecting DBP precursors and bromide from low turbidity surface waters in Colorado than was either UF or microfiltration (MF). Similar observations were reported by Chellam, S. et al., Effect Operating Conditions and Pretreatment for Nanofiltration of Surface Water. In Proc. 1997 Membrane Technology Conf., New Orleans, La., pp. 215-231 (1997a), Allgeier and Summers, Evaluating NF for DBP Control with the RBSMT. Jour. AWWA., 87:3:87-99 (1995), Taylor et al., (Ibid; 1987) and by Lozier and Carlson, Organics Removal from Eastern U.S. Surface Waters Using Ultra-Low Pressure Membranes. In Proc. 1991 Membrane Technologies in the Water Industry, Orlando, Fla., pp. 521-543 (1991) when using ultra-low pressure membrane processes. Using a NF membrane to treat Colorado River water, Amy et al., Removal of Dissolved Organic Matter by Nanofiltration. Jour. Of Environmental Engineering-ASCE., 116:1:200-205 (1990) observed a 65 to 70% reduction in trihalomethane formation potential (THMFP). Mulford et al., NF Performance at Full and Pilot Scale. Jour. AWWA., 91:6:64-75 (1999) used MF or UF before NF to treat high TOC Floridian groundwater. High removal efficiencies of TOC, simulated distribution-THMFP (SDSTHMFP), simulated distribution-HAAFP (SDSHAAFP), and chlorine demand were achieved. Chellam et al., (Ibid; 1997a) observed that when water from the Occoquan Reservoir in Virginia was treated by MF followed by NF, all current and anticipated THM and HAA regulations were met. DiGiano, F. A., et al., Nanofiltration Fouling by Natural Organic Matter and Role of Particles in Flux Enhancement. In Proc. 1993 Membrane Technology Conf., Baltimore, Md., pp. 273-291 (1993) suggested that pretreatment of the raw water before NF would be necessary in order to remove THM precursors effectively if the concentrations of THMFP in the raw water were higher than 100 μg/L.
In the vast majority of studies conducted, organic polymeric membranes were used. Although ceramic membranes are much more resistant to chemical oxidation and extreme temperature (Baker, R. W., Membrane Technology and Application. McGraw-Hill, New York (2000), their use in water treatment is relatively new. Wiesner et al., Permeation Behavior and Filtrate Quality of Tubular Ceramic Membranes Used for Surface Water Treatment. In Proc. 1991 Membrane Technologies in the Water Industry, Orlando, Fla., pp. 371-383 (1991) reported large removals of turbidity, UV-254, TOC and THMFP when using ceramic MF membranes (having pore diameters between 0.1 and 10 μm), with and without coagulation, as a pretreatment step for waters with moderate to high turbidities. Similar results were observed by Scanlan et al., Membrane Filtration for the Removal of Color and TOC from Surface Water. In Proc. 1997 Membrane Technology Conf., New Orleans, La., pp. 127-141 (1997), when treating a low turbidity water, they found that UF was more effective at particle removal than MF, with or without chemical addition; however, the operation times of MF were much longer than those of UF. Lee et al., Evaluation of Ceramic Membrane Application for Drinking Water Treatment Based on NOM/Membrane Characteristics. In 2000 Annual AWWA Conference, Denver, Colo., USA (2000) found that the membranes with similar MWCO values, ceramic membranes were more efficient at removing NOM, HAAFP, and had higher permeability than polymeric membranes. The reduced fouling on ceramic membranes may be the result of the more hydrophilic nature of these membranes.
One of the major problems with membrane processes is the decrease in permeate flux due to membrane fouling. The fouling rates are influenced by the nature of the solutes, their concentrations, membrane type and pore size distribution, water quality, hydrodynamics and the surface characteristics of the membrane (e.g., see Cho et al., (Ibid; 2002); Lee et al., (Ibid; 2001); Field, R. W., (Ibid; 1996). Membrane fouling lowers the economic efficiency of membrane treatment by reducing the quality of treated water, shortens membrane life, and increases the frequency of membrane cleaning. The “fouled” state of the membrane and the characteristics of the foulants are believed to control the rejection of other ions by the membrane (Schafer, Fane and Waite, Fouling Effects on Rejection in the Membrane Filtration of Natural Waters. Desalination, 131:215-224 (2000).
Organic matter is often found to be a primary source of flux decline due to fouling of membrane systems (e.g., see Lee et al., (Ibid; 2001); Nilson and DiGiano, Influence of NOM Composition on Nanofiltration. Jour. AWWA., 88:5:53-66 (1996); Ravindran et al., Crossflow Membrane Filtration for the Removal of Natural Organic Matter. In Proc. 1993 Membrane Technology Conf., Baltimore, Md., pp. 587-599 (1993)). Fouling may be caused by the interaction of NOM with membrane surfaces or incorporation of the NOM into its porous support. DiGiano et al., Fouling of Nanofiltration Membranes by Natural Organic Matter. In Proc. 1994 ASCE Natl. Conf. On Envir. Engr., Boulder, Colo. (1994) attributed the fouling of UF membranes to TOC with a molecular weight greater than 30,000 Da. Similar results were found by Lin et al., Effects of Humic Substance Characteristics on UF Performance. Wat. Res. 34:1097-1106 (2000), although they found that molecules in the molecular weight range of 6,500 to 22,600 Da caused the worst fouling. Aoustin et al., Ultrafiltration of Natural Organic Matter. Sep. Purif. Technol. 22-23:63-78 (2001) found that the larger and more UV-absorbing fraction of humic acid was responsible for irreversible pore adsorption and plugging. Similar results were observed by Bonner et al., Some Aspects of the Fouling of Ultrafiltration Membranes by Natural Organic Matter in Water Treatment. In Proc. 1991 Membrane Technologies in the Water Industry, Orlando, Fla., pp. 239-251 (1991) when using hydrophilic membranes. The hydrophobicity of the organic substances in the treated water is also important in determining fouling behavior (Cho et al., Membrane Filtration of Natural Organic Matter: Comparison of Flux Decline, NOM Rejection, and Foulants During Filtration with Three UF Membranes. Desalination, 127:283-298 (2000a), Schafer et al., Nanofiltration of Natural Organic Matter: Removal, Fouling and the Influence of Multivalent Ions. Desalination, 118:109-122 (1998).
Uncharged fractions of NOM were found to significantly foul negatively charged membranes (Cho et al., Ibid; 2000a). Roudman and DiGiano, Surface Energy of Experimental and Commercial Nanofiltration Membranes: Effects of Wetting and Natural Organic Matter Fouling. J. Membrane Sci., 175:61-73 (2000) found that the surfaces of three experimental thin-film composite nanofiltration membranes became more hydrophilic due to wetting by the permeate water. Unfortunately, the new polymeric materials were not more resistant to fouling than the commercial membranes and in all cases fouling was irreversible. Fan et al, Influence of the Characteristics of Natural Organic Matter on the Fouling of Microfiltration Membranes. Wat. Res., 35:4455-4463 (2001) found that the fouling rate for a hydrophobic membrane was considerably greater than that for a hydrophilic membrane.
Although, pretreatment of raw water (use of chemical clarification or using MF or UF before NF can reduce fouling (e.g., Carroll et al., The Fouling of Microfiltration Membranes by NOM After Coagulation Treatment. Wat. Res., 34:11:2861-2868 (2000); Siddiqui et al., (Ibid; 2000); Lin et al., Ultrafiltration Processes for Removing Humic Substances: Effect of Molecular Weight Fractions and PAC Treatment. Wat. Res., 33:1252-1264 (1999); Chellam et al., Effect of Pretreatment on Surface Water Nanofiltration. Jour. AWWA., 89:10:77-89 (1997b); Amy et al., Membrane Separation of DBP Precursors from Low-Turbidity Surface Waters. In Proc. 1993 Membrane Technology Conf., Baltimore, Md., pp. 651-663 (1993); Moulin et al., Potanilisation of Surface Waters by Crossflow Ultra- and Microfiltration on Mineral Membranes: Interest of Ozone. In Proc. 1991 Membrane Technologies in the Water Industry, Orlando, Fla., pp. 729-737 (1991); Laine et al., Ultrafiltration of Lake Water: Effects of Pretreatment on Organic Partitioning, THM Formation Potential, and Flux. Jour. AWWA, 82:12:82-87 (1990)), these methods are costly and have not always met the needs of the industry. As such, it is important that other processes be combined with membrane filtration to enhance its performance.
Ozonation
Ozone is a powerful oxidant and preferentially oxidizes electron-rich moieties which contain carbon-carbon double bonds, and aromatic alcohols (Bablon et al., Fundamental Aspects. In Ozone in Water Treatment: Application and Engineering. Ed. By Langlais, B.: Reckhow, D. A.: Brink, D. R. Lewis Publishers, Chelsea, Miss. pp. 11-132 (1991)). Ozonation reactions in aqueous solution involve either direct reactions with molecular ozone or indirect ones with the hydroxyl radical, OH. The decomposition reaction is catalyzed by hydroxide ions and other dissolved compounds such as NOM. OH radicals can react with dissolved organic carbon (DOC) and accelerate the decomposition of ozone (Bablon et al., (Ibid; 1991); Staehelin and Hoigne, Decomposition of Ozone in Water in the Presence of Organic Solutes Acting as Promoters and Inhibitors of Radical Chain Reactions. Environ. Sci. Technol., 19:12:1206-1213 (1985); Hoigne and Bader, Ozonation of Water: Role of Hydroxyl Radicals as Oxidizing Agents. Science, 190:782-784 (1975)). The reaction of organic compounds with hydroxyl radicals will produce organic free radicals that ultimately result in the formation of aldehydes, ketones, alcohols, and carboxylic acids. Some of the aldehydes including formaldehyde, acetaldehyde, glyoxal, and methylglyoxal are of particular concern due to their mutagenicity and carcinogenicity (Richardson, S. D., Drinking Water Disinfection By-products. In: Encyclopedia of Environmental Analysis and Remediation, R. A. Meyers, Ed. New York: John Wiley & Sons, Inc. (1998); Bull and McCabe, Risk Assessment Issues in Evaluating the Health Effects of Alternate Means of Drinking Water Disinfection. In Water Chlorination: Chemistry, Environmental Impact, and Health effects, Vol. 5, Edited by Jolley, R. L. et al., Lewis Publishers, Chelsea, Mich. (1984)).
Molecular ozone and OH radical reactions, both of which occur during ozonation, can result in the cleavage of larger molecules. This cleavage results in lower molecular weight material and the formation of more polar and hydrophilic compounds (e.g. see: Koechling et al., Effect of Ozonation and Biotreatment on Molecular Size and Hydrophilic Fractions of Natural Organic Matter. In Water Disinfection and Natural Organic Matter: Characterization and Control. ACS Symposium Series 649: 196-210 (1996); Owen et al., NOM Characterization and Treatability. Jour. AWWA., 87:1:46-63 (1995); Amy et al., Molecular Size Distribution of Dissolved Organic Matter. Jour. AWWA., 84:6:67-75 (1992)). A decrease in the concentration of UV absorbing compounds of NOM was also observed during ozonation (Yavich, A. A., The Use of Ozonation and Biological Fluidized Bed Treatment for the Control of NOM in Drinking Water. Ph.D. Dissertation. Michigan State University. (1998); Koechling et al., (Ibid; 1996); Shukairy et al., Bromide's Effect on DBP Formation, Speciation, and Control: Part 1, Ozonation. Jour. AWWA., 86:6:72-87 (1994); Kaastrup and Halmo, Removal of Aquatic Humus by Ozonation and Activated Carbon Adsorption. In Aquatic Humic Substances: Influence on Fate and Treatment of Pollutants. I. H. Suffet, and P. MacCarthy, Eds. American Chemical Society, Washington, D.C., pp. 697-726 (1989); Amy et al., Ozonation of Humic Substances: Effects on Molecular Weight Distributions of Organic Carbon and Trihalomethane Formation Potential. Ozone Sci. Eng., 10:39-54 (1988)).
Replacing chlorination with ozonation as the primary disinfectant can significantly reduce the formation of THMs and HAAs (Zhang et al., Characterization and Comparison of Disinfection By-products of Four Major Disinfectants. In Natural Organic Matter and Disinfection By-Products: Characterization and Control in Drinking Water, Edited by Barrett, S. E. et al., American Chemical Society, Washington D.C. (2000); Richardson et al, Identification of New Ozone Disinfection By products in Drinking Water. Environ. Sci. Technol. 33:19:3368-3377 (1999)). In the presence of NOM, ozonation results in the formation of partial oxidized compounds, which are less reactive with chlorine in forming THMs (Amy et al., (Ibid; 1988). Chang et al. Reducing the Formation of Disinfection By-products by Preozonation. Chemosphere, 46:21-30 (2002) found that both pre- and post-ozonation processes can reduce some of DBP precursors and overall DBP-formation potential (DBPFP) more than the conventional drinking water treatment process. They applied the pre-ozonation process to treat drinking water and observed a reduction of 9-54% in DOC and more than 40% of DBPs. The required ozone dosage and the formation of aldehydes and ketoacids increased with increasing NOM concentration (Najm et al., Effects of Bromide and NOM on By-product Formation. Jour. AWWA., 87:1:106-115 (1995)). As the ozone dosage increased, THM and HAA formation decreased (Cipparone et al., Ozonation and BDOC Removal: Effect of Water Quality. Jour. AWWA., 89:2:84-97 (1997)). The ozonation of water containing humic substances results in the formation of hydroxyl, carbonyl, and carboxyl groups, and aliphatic and alicyclic ketones (Glaze et al., Evaluation of Ozonation By-products from Two California Surface Waters. Jour. AWWA., 81:8:66-73 (1989); Anderson et al., The Reaction of Ozone with Isolated Aquatic Fluvic Acid. Org. Geochem., 8:1:65-69 (1985)). Gracia et al, Study of the Catalytic Ozonation of Humic Substances in Water and Their Ozonation By products. Ozone Sci. Eng., 18:3:195-208 (1996) obtained similar results. They identified 110 different organic compounds resulting from ozonation of humic substances.
Catalytic Ozonation
The use of catalytic ozonation for the degradation of NOM and other organic compounds in drinking water and wastewater is a promising technology (Legube and Karpel Vel Leitner, N. Catalytic ozonation: A Promising Advanced Oxidation Technology for Water Treatment. Catal. Today, 53:61-72 (1999). For example, Gracia, R.; Cortes, S.; Sarasa, J.; Ormad, P; Ovelleiro, J. L., Catalytic Ozonation with Supported Titanium Dioxide. The Stability of Catalyst in Water. Ozone Sci. Eng., 22: 185-193 (2000); Gracia, R.; Cortes, S.; Sarasa, J.; Ormad, P; Ovelleiro, J. L., Heterogeneous Catalytic Ozonation with Supported Titanium Dioxide in Model and Natural Waters. Ozone Sci. Eng., 22:461-471 (2000); and Gracia, R. et al., (Ibid; 1996) found that the removal of NOM and a model humic acid was significantly greater in the presence of a catalyst than with ozone alone. Legube and Karpel Vel Leitner et al. (Ibid; 1999) reported that attapulgite (a clay-like material) catalyzed the degradation of humic substances by ozone. The mechanism of the catalytic reaction is poorly understood, but it is thought that, at least in some cases, the metal oxide surface initiates the decomposition of ozone and that produces .OH or .02− radicals which degrade sorbed organic compounds (Legube and Karpel Vel Leitner, (Ibid; 1999). Li, W.; Gibbs, G. V.; Oyama, S. T. Mechanism of Ozone Decomposition on a Manganese Oxide Catalyst. 1. In Situ Raman Spectroscopy and ab initio Molecular Orbital Calculations. J. Am. Chem. Soc., 120:9041-9046 (1998) found that the decomposition of ozone on manganese resulted in the formation of a peroxide specie. It is not known if this peroxide is responsible for the degradation of sorbed organic compounds. Other possible mechanisms are discussed by Legube and Karpel Vel Leitner (Ibid; 1999).
As is discussed below, catalytic ozonation may reduce membrane fouling problems by degrading organic foulants sorbed on or trapped near the membrane surface. Several metal oxides that are used in the fabrication of ceramic membranes catalyze ozone decomposition. Titania catalyzes ozone decomposition and it also catalyzes the removal of NOM and other organic compounds in the presence of ozone (e.g., Gracia et al., (Ibid; 2000a and 2000b). Alumina and zirconia have been shown to promote the decomposition of ozone (Radhakrishnan, R. and Oyama, S. T.; Chen, J. G. G.; Asakura, K. Electron transfer Effects in Ozone Decomposition on Supported Manganese Oxide. J. Phys. Chem. B, 105:4245-4253 (2001); Legube and Karpel Vel Leitner (Ibid; 1999). γ-alumina is reported to catalyze the degradation of 2-chlorophenol by ozone (Ni, C. H.; and Chen, J. N. Heterogeneous Catalytic Ozonation of 2-chlorophenol Aqueous Solution with Alumina as a Catalyst. Water Sci. Technol., 43:213-220 (2001). An iron coated alumina catalyzed the degradation of phenol by ozone (Al Hayek, N.; Legube, B.; Dore, M. Ozonation Catalytique (FeIII/Al2O3) du phenol et de ses produits d'ozontion. Environ. Technol. Lett., 10:415-426 (1989). To our knowledge, there are no reports in the literature of zirconia catalyzing the reaction of ozone with NOM or similar organic compounds.
Manganese dioxide and iron oxide, which are not commonly used for the fabrication of ceramic membranes, are known to catalyze the degradation of various organic compounds in the presence of ozone (e.g., Lim, H. N.,; Choi, H.; Hwang, T. M.; Kang, J. W. Characterization of Ozone Decomposition in a Soil Slurry. Water Res., 36:219-229 (2002); Andreozzi, R.; Caprio, V. Marotta, R. Tufano, V. Kinetic Modeling of Pyruvic Cid Ozonation in Aqueous Solution Catalyzed by Mn(II) and Mn(IV) ions. Water Res., 35:109-120 (2001); Choi, H.; Kim, Y. Y.; Cho, J.; Kang, J. W.; Kim, K. S. Oxidation of Polycyclic Aromatic hydrocarbons by Ozone in the Presence of Sand. Water Sci. Technol.; 43:349-356 (2001); Radhakrishmnan, R.; Oyama, S. T., Ozone Decomposition over Manganese Oxide Supported on ZrO2 and TiO2: A Kinetic Study using in situ Laser Raman Spectroscopy. J. Catal., 199:282-290 (2001); Ma, J. and Graham, N. J. D., Degradation of Atrazine by Manganese-catalysed ozonation: Influence of Humic Substances. Water Res., 33:785-793 (1999); Li, W.; Gibbs, G. V.; Oyama, S. T. (Ibid; 1998); Li, W.; Oyama, S. T. Mechanism of Ozone Decomposition on a Manganese Oxide Catalyst. 2. Steady-state and Transient Kinetic Studies. J. Am. Chem. Soc., 120:9047-9052 (1998); Masten, S. J. and Davies, S. H. R., Efficacy of In-situ Ozonation for the Remediation of PAH Contaminated Soils. J. Contam. Hydrol. 28:327-335 (1997) and Andreozzi, R.; Caprio, V., D'Amore, M. G.; Insola, V. Manganese Catalysis in Water Pollutants Abatement by Ozone. Environ. Technol., 16:885-891 (1995). Manganese dioxide is a particularly effective catalyst and it is used in at least one commercial catalytic ozonation system (manufactured by NGK Insulators Inc., Tokyo, Japan).
Ozonation/Membrane Filtration
Few researchers have investigated the combination of ozonation and membrane processes. Unfortunately, organic membranes are prone to destruction by ozone (Shanbag, P. V., Guha, A. K., Sirkar, K. K. Membrane-Based Ozonation of Organic Compounds. Ind. Eng. Chem. Res., 34:11:4388-4398 (1998); Castro, K., Zander, A. K., Membrane Air-stripping: Effects of Pretreatment. Jour. AWWA., 87:3:50-61 (1995); and Shen, Z., Semmens, J. J. A novel Approach to Ozone—Water Mass Transfer Using Hollow—Fiber Reactors. Environ. Tech., 11:597-608 (1990). In order to prevent damage to the membrane, a retention or aeration tank is often used between ozonation and the membrane module, allowing for stripping of residual ozone. Hyung, H.; Lee, S., Yoon, J.; Lee, C.-H. Effect of Preozonation on flux and Water Quality in Ozonation-Ultrafiltration Hybrid System for Water Treatment. Ozone Sci. Eng., 22:637-652 (2000) investigated the effect of ozonation on membrane flux and water quality in an ozonation/UF hybrid system using water from two unidentified locations in the Han River. Ozonated water was retained for one hour before being used as feed water to the UF. They found that membrane flux varied inconsistently with the quality of water. About 22% of TOC, 64% of UV-254, and 36-53% of THM precursors were removed by this hybrid system. (O'Connell, J.; Danos, S. An Innovative Combination of Ozonation and Ultrafiltration. In Proc. 1997 Membrane Technologies Conf., New Orleans, La., pp. 1127-1145 (1997) used a similar system to treat well water with elevated levels of iron and manganese. High removal rates, 97% of the iron and 91% of the manganese, were achieved and the quality of permeate remained consistent, despite the fluctuation of feed quality.
Ozone-resistant membranes may be used to avoid the problem of membrane oxidation. (Hashino, M.; Mori, Y.; Fujii, Y.; Motoyama, N.; Kadokawa, N.; Hoshikawa, H.; Nishijima, W.; Okada, J. Pilot Plant Evaluation of an Ozone-Microfiltration System for Drinking Water Treatment. Water Sci. and Tech., 41:10-11:17-23 (2000) reported, when using an ozone resistant polyvinylidenefluoride (PVDF) MF membrane, that high dissolved ozone concentrations on the membrane surface were necessary to obtain high permeate fluxes and to prevent membrane fouling. Ceramic membranes in combination with ozonation achieved a high permeate flux without membrane damage (Kim, J. O.; Somiya, I. Effective Combination of Microfiltration and Intermittent Ozonation for High Permeation Flux and VFAs Recovery from Coagulated Raw Sludge. Environ. Technol., 22:7-15 (2001); Kim, J.-O., Somiya, I.; Fujii, S. Fouling Control of Cermaic Membrane in Organic Acid Fermenter by Intermittent Ozonation. In Proceedings of the 14th Ozone World Congress. Vol. 1, pp. 131-143, Dearborn, Mich. (1999); Moulin, C. et al., (Ibid; 1991). Kim et al (Ibid; 1999) showed that intermittent ozonation is effective for maintaining high permeation flux and prevents membrane fouling caused by particle accumulation on the membrane surface. These studies demonstrate the potential of ozone to reduce membrane fouling. The potential exists to decrease membrane fouling further or reduce ozone dosages by the use of a catalytic membrane to selectively oxidize the organic material deposited at the membrane surface.
Catalytic Membranes
The use of catalytic membranes for synthesis and for the treatment of waste streams is an emerging technology (Coronas, J.; Santamaria, J. Catalytic Reactors based on Porous Ceramic Membranes. Catal. Today, 51:377-389 (1999). Catalytic membranes may be useful for the degradation of membrane foulants, as the possibility of surface chemical reactions occurring with the sorbed or trapped species is greater than for substances that are in the bulk water phase. Liu, P., Wang, X. C., Fu, X. Z., Processing and Properties of Photocatalytic Self-cleaning Ceramic. J. Inorg. Materials, 15:88-92 (2002) described the use of a “self-cleaning” membrane for the filtration of oleic acid solutions. The membrane was prepared by coating a photocatalytic membrane on a ceramic matrix. Tsuru, T., Toyosada, T., Yoshioka, T., Asaeda, J., Photocatalytic Reactions in a Filtration System through Porous Titanium Dioxide Membranes. J. Chem. Eng. Japan, 34:844-847 (2001) also found that membrane fouling could be reduced by the photocatalytic degradation of organic foulants. In a model system containing 500 ppm polyethyleneimine the volume flux increased two-fold when the membrane was illuminated and then decreased to nearly its original level when the light was turned off. These studies show the potential that radical species generated at or near the membrane surface could significantly reduce fouling problems in membrane filtration systems. The advantages of the ozone system proposed over a photocatalytic membrane are that the geometry of the membrane module is not constrained by the need to irradiate the membrane surface and in the ozone system the degradation of NOM can also occur in the bulk water.
Membrane Properties and Membrane Performance
The electrokinetic properties of NF membranes have a great influence on permeate flux, solute rejection and the fouling properties of the membrane. The flux through a membrane is influenced by surface charge due to the “electro-viscous effect”. The electro-viscous effect arises due to an apparent increase in the viscosity of the electrolyte solution due to liquid flow induced by the conduction current, in the opposite direction to the pressure flow (Erickson, D., Li, D., Streaming Potential and Streaming Current Methods for Characterizing Heterogeneous Solid Surfaces. J. Colloid Interfac. Sci., 237:283-289 (2001). The flux through NF membranes is greatest when the charge on the membrane surface is near zero (e.g., Tsuru, T., Hironaka, D., Yoshioka, T., Asaeda, M., Titania Membranes for Liquid Phase Separation: Effect of Surface Charge on Flux. Sep. Purif. Technol. 25:307-314 (2001); Childress, A. E., Elimelech, M., Relating Nanofiltration Membrane Performance to membrane Charge (Electrokinetic). Characteristics. Environ. Sci. Technol., 34:3710-3716 (2000); and Huisman, I. H., Tragardh, G., Tragardh, C., Pihlajamaki, A., Determining the Zeta-potential of Ceramic Microfiltration Membranes using the Electroviscous Effect. J. Membr. Sci., 147:187-194 (1998). The rejection of charged species by the membrane is also strongly affected by surface charge (Childress and Elimelech, (Ibid; 2000); and Cho, J., Amy, G., Pellegrino, J., Membrane Filtration of Natural Organic Matter: Factors and Mechanisms Affecting Rejection and Flux Decline with Charged Ultrafiltration (UF), membrane. J. Membr. Sci., 164:89-110 (2000). The fouling of the membrane is influenced by electrostatic repulsion effects between the NOM and the membrane surface (Seidel, A., Elimelech, M., Coupling between Chemical and Physical Interactions in Natural Organic Matter (NOM) fouling of Nanofiltration Membranes: Implications for Fouling Control. J. Membr. Sci., 203:245-255 (2002); Cho et al., (Ibid; 2000)). Other surface properties may influence membrane performance. As mentioned previously, hydrophobic membranes are generally more prone to fouling by natural organic material than are hydrophilic membranes.
Membrane surface charge is influenced not only by the properties of the membrane material but also by the solution chemistry of the water being filtered. Sorption of charge species particularly, divalent cations, such as Ca2+, have a pronounced influence on membrane surface charge and performance (e.g., Tay, J.-H., Liu, J., Sun, D. D., Effect of Solution Physico-chemistry on the Charge Property of Nanofiltration Membranes. Water Res., 36:585-598 (2002); Cho et al., (Ibid; 2000); and Childress, A. E., Elimelech, M., Effect of Solution Chemistry on the Surface Charge of Polymeric Reverse Osmosis and Nanofiltration Membranes. J. Membr. Sci., 119:253-268 (1996)).
In a catalytic membrane, the catalyst will alter the surface charge of the membrane. A catalytic filtration layer may also alter both the surface charge and the permeability of the membrane. A thin coating of the catalyst would alter the surface charge of the membrane, if the acid-base properties of the catalyst and underlying membrane material were different. However, due to the limited thickness of the catalytic coating, it probably would not have a pronounced effect on the permeate flux or streaming potential of the membrane (Szymczyk, A., Fievet, P., Reggiani, J. C., Pagetti, J., Determination of the Filtering Layer Electrokinetic Properties of a Multi-layer Ceramic Membrane. Desalination, 116:81-88 (1998).